A wafer boat is typically used to hold a plurality of semiconductor wafers for processing in conventional vertical heat treatment apparatus. The wafer boat commonly comprises a plurality of support accommodations that support the wafers at edge portions of those wafers. In this way, the wafers are held oriented horizontally in a vertically spaced manner. In general, wafers are automatically loaded from a wafer container into a boat using a generic wafer handler, such as by contacting the wafer at its backside or bottom surface, with an end effector.
For high ramp rate heat treatment processes and CVD processes, each support accommodation may comprise a wafer holder that underlies an entire perimeter of a wafer. Such a wafer holder can be a plate or can be in the shape of a ring. Such a wafer holder helps to minimize non-uniformities in the temperature distribution on the surface of a wafer supported on that wafer holder and, desirably, thereby minimizes non-uniformities in the thickness of various films formed on the wafer or non-uniformities of other high temperature treatment effects.
Automatically loading wafers into a wafer boat having wafer holders that extend in an unbroken line around an entire perimeter of a wafer is problematic, because generic wafer handlers are typically not able to load a wafer onto such wafer holders. For example, generic wafer handlers typically carry a wafer on an end effector, which typically contacts a wafer at its edges or at a central area in the wafer's backside. In order to seat a wafer onto a wafer holder, the end effector typically must pass through the outer perimeter of the wafer holder to center and set the wafer on the wafer holder. Wafer holders that are plates or complete rings, however, do not have breaks or openings in their perimeters to allow end effectors to pass into their centers and, so, prevent the end effectors from being able to center and place wafers upon the wafer holders. To solve this specific problem, several loading apparatuses and methods have been suggested in the prior art.
In U.S. Pat. No. 5,162,047 to Wada et al., a method and wafer transfer apparatus is described for transferring wafers one-by-one from a wafer cassette to a wafer boat having wafer holders that are complete rings. The wafer transfer apparatus comprises a lower assembly and an upper assembly. The lower assembly has a lift mechanism to vertically move the upper assembly. The upper assembly has mounted on it an upper slider and a lower slider, each moveable in a horizontal direction. A receiver, provided with support pins, is attached to the distal end of the lower slider and a conveying arm is attached to the distal end of the upper slider. The receiver attached to the lower slider is vertically movable.
FIG. 1 shows the method described by Wada et al. for transferring a semiconductor wafer 300 from a wafer cassette (not shown) to a support ring 302 in a wafer boat (not shown). First, a semiconductor wafer is picked-up, in a substantially horizontal state, from the wafer cassette by the conveying arm 304, and positioned above a support ring 302 inside the vertical boat, as shown in FIG. 1A. Subsequently, the receiver 306 is positioned below the support ring 302, and then moved upwardly to a position where support pins 308 of the receiver 306 reach the conveying arm 304 through an central opening 310 in the support ring 302, as shown in FIG. 1B. The wafer 300 is lifted from the conveying arm by the support pins 308. Next, the conveying arm 304 is retracted from the boat, as shown in FIG. 1C. Finally, the receiver 306 is moved downwardly to a position where the wafer 306 is transferred to the support ring 302, and the receiver 300 is also retracted from the boat, as shown in FIG. 1D. Progressing from one wafer holder 302 to another, the sequence of events described above is repeated until the wafer boat is fully loaded with wafers 300. Unloading of the wafers 300 from the wafer boat can be performed by the reverse of the procedure described above.
A disadvantage of this method is that the time required to execute the movements of the receiver must be added to the wafer transfer time. Because the wafers are both transferred to the wafer boat one at a time and also seated upon the wafer holders one at a time, the total time to load or unload a boat is significant. To reduce the loading/unloading time per batch treatment cycle, Wada et al. suggest converting the wafer transfer apparatus into an apparatus in which two or more semiconductor wafers can be transferred simultaneously. The apparatus consists of a multistage arrangement of identical apparatuses to transfer a plurality of wafers. While this can lead to some reduction of the loading/unloading time per batch treatment cycle, it is, undesirably, a costly solution involving a complex wafer transfer apparatus.
Another apparatus for simultaneously transferring two or more wafers into a wafer boat having ring-type wafer holders is described in U.S. Pat. No. 5,334,257 to Nishi et al. As shown in FIG. 2, Nishi et al. describes a wafer boat 200 that comprises a plurality of fixed ring-shaped wafer holders 202. Each ring-shaped wafer holder 202 is provided with at least three cutout sections 204 located at the outer edge of the ring 202. The cutouts are of a size and shape sufficiently large to allow passage of supporting teeth 206, which can temporarily hold wafers (not shown) inside the wafer boat 200 over the fixed wafer holders 202. The supporting teeth 206 are provided at the distal ends of supporting arms 208, which are in turn mounted to a supporting tooth drive device 210. Each of the supporting teeth 206 can freely enter and leave one of the cutout sections 204 by moving in a horizontal direction A, B, or C, respectively. In order to load a plurality of wafers at one time in the wafer boat 200, Nishi suggests in an alternative embodiment to provide each supporting arm 208 at its distal end not with only a single support tooth 206 but with a support comb (not shown), which comprises a plurality of vertically spaced apart support teeth 206 with vertical spacing corresponding with the spacing of the wafer holders 202 along the height of the wafer boat. Using the support comb with the plurality of support teeth 206, it is possible to load up to 30 semiconductor wafers (not shown) at one time in the wafer boat 200.
Batch-wise loading of the wafer boat comprises the following steps. First, the support comb is inserted into the cutouts such that the support teeth 206 are positioned for supporting the wafers (not shown) spaced above the wafer holders 202, without touching the wafer holders 202. Then the wafers are transferred from a wafer cassette 212 to a position inside the wafer boat using a fork-shaped transfer device 214, shown schematically. The supporting teeth 206 temporarily support the wafers above the wafer holders 202, without touching the wafer holders 202. After the capacity of the support comb is reached, e.g., after a maximum of 30 wafers is supported on the support teeth 206, the wafers are loaded onto the ring-shaped wafer holders 202 by changing the vertical position of the wafer boat 200 relative to the supporting teeth 206. By raising the boat 200 while the support teeth 206 remain stationary, the support comb is shifted in a downward direction relative to the wafer holders 202. By this movement, each support tooth 206 passes below a wafer support surface of the wafer holder 202 and the wafer above it is handed off onto the wafer holder 202. Depending on the number of wafer accommodations in a wafer boat 200 and the number of support teeth 206, a boat 200 can be completely loaded by performing this procedure one or more times. Unloading of the wafers from the wafer boat 200 can be accomplished by performing the reverse of the procedure described above.
Although the apparatus described by Nishi et al. leads to a reduction in the exchange time per treatment cycle, its implementation is undesirably complex and space consuming. Furthermore, it requires cut-out 204 at the outer perimeters of the wafer holders for the teeth 206. These cut-outs 204 leave exposed parts of the backside of wafers seated upon the wafer holders 202 and, so, may cause temperature non-uniformities in the wafers and may also influence the quality of layers deposited on those wafers.
Accordingly, it is an object of the present invention to provide an apparatus and method capable of efficiently loading a plurality of wafers into a wafer boat having wafer holders that support an entire perimeter of each of the plurality of wafers.